

下列整式中,单项式是________;多项式是 ________.
\(a,\)\(\frac{2}{5}x - b{y}^{3}, \)\(- \frac{1}{3}{x}^{2}y,\)\({2\pi r},\)\({x}^{2} + {xy} + {y}^{2},\)\({2x} - 1\) .
已知x2+x-1=0,求x3+2x2+3的值.
解:x3+2x2 +3=x3+x2-x+x2+x+3
=x(x2+x-1)+x2+x-1+4
=0+0+ 4=4
如果1+x +x2+x3=0,求x+x2+x3+x4+x5+ x6+x7+x8的值.